In a number of manufacturing operations involving the use of continuous filaments of material, it is desirable to apply various liquid additives to the filaments to provide useful benefits in connection with such manufacturing operations. The liquid additives may be lubricants, sizing solutions, finish compositions, plasticizers, etc. which impart desired characteristics to the treated filaments in a product manufactured therefrom or which provide some degree of protection to the filaments as they are being processed in the manufacturing operations. Since many manufacturing operations employ a rope of tow consisting of a large number of individual filaments, the uniform application of liquid additives to filaments making up the rope or tow is difficult to achieve.
One particular manufacturing operation in which multifilament tows are used is the production of filter rods for smoking products such as cigarettes. Filaments of cellulose acetate are typically gathered into a rope or tow and the tow is then treated with desired amounts of a plasticizer and/or other suitable additives before forming the treated cellulose acetate into a continuous filter rod that is subsequently cut to desired lengths. The plasticizer applied to the tow improves the firmness of the formed filter rod with the firmness being dependent on the extent to which the individual filaments in the tow have been coated with plasticizer. Two basic methods are used at the present time to apply plasticizer to filter tow. One method utilizes spraying or similar devices for applying plasticizer to both sides of a flattened, spread out band of filter tow. The other method involves contacting each side of a flattened band of tow with a surface that is continuously wetted with plasticizer. The latter method may employ either rotating surfaces such as roller devices or stationary surfaces across which the band of tow moves.
Disclosed in U.S. Pat. No. 3,157,536 is apparatus which employs two cooperating applicators for applying plasticizer to filter tow. Each applicator includes a stainless steel tube that is provided with a narrow slit longitudinally disposed along one side of the tube. Stainless steel mesh or screen fabric concentrically surrounds the tube and a porous felt material is interposed between the steel tube and steel screen so that the narrow slit is completely covered by the felt. Plasticizer is forced under pressure into the tube from which it exits through the narrow slit, the porous felt material and the overlying steel mesh or screen fabric. A continuous filter tow is brought into contact with the applicators so that one side of a flat band of the tow is pulled across the wire screen of one applicator and the opposite side of the band of tow is pulled across the wire screen of the second applicator. Although the disclosed applicators are effective for the treatment of filter tow, they do have certain disadvantages associated therewith. For example, the applicators require a 16 hour "break-in" period and they undergo sufficient wear during use to make replacement necessary after a few months' use. Also, the applicators must be dismantled frequently for cleaning purposes and great care must be exercised during the cleaning operation to avoid damage to the wire screen. Slight abrasions to the tow-contacting surface of the wire screen will lead to excessive generation of lint in the filter tow and the filter rod formed from the tow. In addition to the undesirable effects on the characteristics of the formed filter rod caused by a damaged wire screen, the time required to perform routine cleaning and maintenance of such applicators is very substantial. Thus, the "down time" associated with the use of the wire screen applicators has a very significant economic impact on a filter rod manufacturing operation which employs such applicatiors.